In recent years, the image recording material is predominated by a material for forming particularly a color image and, specifically, a recording material for an inkjet system, a recording material for a heat transfer system, a recording material for an electrophotographic system, a silver halide light-sensitive material of transfer type, a printing ink, a recording pen and the like are being actively utilized.
The inkjet recording method is abruptly spread and still making a progress because of low material cost and capability of high-speed recording, low-noise recording and easy color recording.
The inkjet recording method includes a continuous system of continuously flying a liquid droplet and an on-demand system of flying a liquid droplet according to image information signals, and the ejection system therefor includes a system of ejecting a liquid droplet by applying a pressure from a piezoelectric element, a system of ejecting a liquid droplet by generating a bubble in the ink under heat, a system using an ultrasonic wave, and a system of suctioning and ejecting a liquid droplet by an electrostatic force. As for the inkjet recording ink, an aqueous ink, an oil-based ink or a solid (fusion-type) ink is used.
The color image formation by an inkjet recording method using a plurality of color ink compositions is performed using three colors of a yellow ink composition, a magenta ink composition and a cyan ink composition or, if desired, using four colors additionally including a black ink composition. In some cases, a color image is formed using six colors including a light cyan ink composition and a light magenta ink composition in addition to the above-described four colors or using seven colors by further adding a dark yellow ink composition. Such a combination of two or more kinds of ink compositions is an ink set.
As regards the ink composition used for the formation of a color image, it is required that, for example, the ink composition of each color has good colorability (high optical density) by itself, a good intermediate color can be formed when ink compositions for a plurality of colors are combined, or the recorded material obtained is kept from discoloration or fading during storage.
The dyestuff used in such an inkjet recording ink is required to exhibit good solubility or dispersibility in a solvent, allow for high-density recording, provide a good color hue, be fast to light, heat and active gases in the environment (for example, an oxidative gas such as NOx and ozone, and SOx), be excellent in the resistance against water and chemicals, ensure good fixing and less blurring on an image-receiving material, give an ink with excellent storability, have high purity and no toxicity, and be available at a low cost.
However, it is very difficult to find out a dyestuff satisfying these requirements at a high level.
Improvement required of the ink set is, in addition to the colorability, fastness and storability of each ink composition constituting the ink set, to form particularly a mixed color portion (for example, a green part, a blue part, a red part and a gray part) that is also excellent in the color hue (the color reproduction region is wide), fastness and storability, assured of high tinctorial strength (the optical density is high) and at the same time, kept from a bronze phenomenon.
However, in the case where a recorded image having a high optical density is formed, there arises a problem that as the image is dried, the dyestuff crystal deposits on the recording material surface and the recorded image reflects light to cause a so-called bronze phenomenon of emitting metallic gloss. This phenomenon is considered to readily occur resulting from increase in the associating property (aggregability) of the dyestuff when the water solubility of the dyestuff is decreased so as to enhance the water resistance or an amino group as a hydrogen bonding group is introduced into the dyestuff structure. Since light is reflected due to generation of a bronze phenomenon, not only the optical density of the recorded image decreases but also the color hue of the recorded image comes to greatly differ from the desired color hue. Accordingly, it is technically difficult to find out an inkjet ink capable of exhibiting an excellent performance in all of bronze phenomenon suppression, color hue, optical density and the like in a single color part and a mixed color part.
As regards the method of suppressing a bronze phenomenon, Patent Document 1 discloses an yellow ink for inkjet recording, where the total amount, in the ink, of cations except for a monovalent metal ion, a hydrogen ion, an ammonium ion, an organic quaternary nitrogen ion and an ion produced resulting from proton addition of a nitrogen atom in a basic organic material is adjusted. Also, Patent Document 2 discloses an ink set for inkjet recording, where the counter cation of an anionic group of a dye contained in the ink set for inkjet recording is changed so as to make it difficult for the printed inks even when mixed to deposit and generate a bronze part. Furthermore, in Patent Document 3, it is disclosed that when the counter cation of an ionic hydrophilic group of a yellow dye contained in a yellow ink for inkjet recording is changed, an effect is obtained in terms of color hue and bronze phenomenon suppression. Particularly, in Patent Document 4, it is disclosed that when a combination of remarkably good yellow, magenta, cyan and black ink colorants is used in a most preferred embodiment as an ink set for inkjet recording, an effect is obtained in terms of bronze phenomenon suppression in each single color (yellow, magenta or cyan) part and a gray part.
In recent years, the ink set is suitably used for the printing of a photographic image and in the formation of a photographic image, it is strongly demanded to satisfy all of the above-described performances required of the water-soluble inkjet recording ink and develop a more excellent ink set. Above all, improving at the same time the bronze phenomenon also in a mixed portion (for example, a green part, a blue part, a red part and a gray part) at a high level is constantly required in view of high image quality.    Patent Document 1: JP-A-2004-123777 (the terms “JP-A” as used herein means an “unexamined published Japanese patent application”    Patent Document 2: JP-A-2004-307819    Patent Document 3: JP-A-2007-63520    Patent Document 4: JP-A-2007-138124